The present invention relates generally to circuit condition monitoring devices for use in electrical power distribution systems, and in particular to a clamp mechanism and housing for mounting such devices to cables of such distribution systems.
Electrical power distribution systems may require the use of a variety of circuit condition monitoring devices to facilitate the detection and location of system malfunctions. Such devices include manually and automatically reset current fault indicators, such as those manufactured by E. 0. Schweitzer Manufacturing Co., and described in U.S. Pat. Nos. 4,288,743, 4,086,529 and 3,676,740 of the present inventor, as well as voltage monitoring devices, such as described in U.S. Pat. Nos. 4,251,770, 4,152,643, 4,263,350 and 4,550,288 of the present inventor. The devices may be either of the test point mounted type for mounting on a system test point, or of the clamp-on type, for clamping directly onto a cable of the system.
Clamp-on type devices typically include a housing containing monitoring circuitry, a circuit condition indicator, and some form of clamp mechanism affixed to the housing for mounting the housing on a cable. Frequently, the clamp mechanism serves the additional function of establishing either magnetic or electrostatic communication, or both, between the electrical conductor in the cable and appropriate sensing components of the monitoring apparatus.
Some clamp mechanisms previously known in the art for mounting distribution system monitoring devices typically required the use of both hands as well as a pair of live-line handling tools such as lineman's hotsticks during installation on a system cable. Consequently, such clamp mechanisms were undesirably difficult and time consuming to install.
In other previously known clamp mechanisms the spacing between the conductor and the sensing element varied substantially as a function of cable diameter, causing the calibration of the monitoring device to undesirably vary with cable diameter. Thus, the user was required to calibrate the device for use with a particular diameter cable.
In one prior clamp mechanism, which is described in U.S. Pat. No. 4,456,873 of the present inventor, a plurality of flexible metallic strips are combined to form a generally square-shaped magnetic core for use with a circuit condition monitoring device. One end of the core is affixed to the device housing, and the opposite end is detachably received within an aperture in the housing opposite the first end. In use, the free end of the core is displaced from the device housing, allowing the core to be passed around a cable. The free end is then inserted in the aperture to lock the device to the cable. A similiar core structure is shown in U.S. Pat. No. 3,725,832 of the present inventor.
In another prior clamp mechanism described in U.S. Pat. No. 4,288,743 two insulated metallic members project from the rear surface of the monitoring device. A third spring-biased member normally occupies a position engaging the two projecting members, but can be displaced from the members to allow a cable to be introduced between the two members. The spring-biased member is then allowed to return to its normal position to clamp the device to the cable. In another prior clamp mechanism described in U.S. Pat. No. 4,646,006 two inwardly biased wire clamps are pre-configured to a spaced-apart condition by a spring under compression prior to receiving the cable. The mechanism is maneuvered with a single hotstick so that the cable deforms the pre-configured spring to cause the clamp to engage the cable.
The present invention is directed to a clamping mechanism and housing whereby a monitoring device can be installed on an electrical cable using a single lineman's hotstick and the calibration of the monitoring circuit does not vary appreciably with cable diameter. The one-hand installation contributes to safety, particularly where it is necessary for the lineman to ascend a ladder in order to install the device, or where the monitored cable is closely spaced to other cables and can only be approached from one direction. Moreover, the mechanism and housing of the invention do not have to be pre-configured to an open condition to receive the cable, since the unique geometry of the clamp members allow the cable to be readily engaged at any time with only an applied force.
Accordingly, it is a general object of the present invention to provide a new and improved clamp mechanism and housing for circuit monitoring devices.
It is a more specific object of the present invention to provide a clamp mechanism and housing for installing a circuit condition monitoring device on the cable of a power distribution system which requires the use of only one hand during installation.
It is a further object of the invention to provide a clamp mechanism and housing for a cable-mounted condition monitoring device wherein the calibration of the monitoring device is substantially independent of cable diameter.
It is a still further object of the invention to provide a clamp mechanism and housing which does not require pre-configuration prior to installation on an electrical cable.